1. Field of the Invention
This invention relates to cold rolled steel sheets and plated steel sheets having considerably improved press formability, phosphatability, weldability and resistance to galling by controlling surface roughness pattern of steel sheet.
2. Related Art Statement
Drawable cold rolled steel sheets used for automobile panels, electric appliances, culinary equipments and so on are required to have an excellent deep drawability. In order to enhance the deep drawability, it is necessary that the steel sheet has high ductility (E1) and Lankford value (r-value) as mechanical properties. In fact, the drawing (particularly in the formation of automobile panel) is frequently combined with the flanging, so that work hardening index (n-value) becomes also important.
In the outer panel for the automobile, the finish feeling after painting is an important item directly connecting to the quality of the automobile itself in users.
Furthermore, the pretreatment for baking or phosphatability is important in the steel sheet for automobiles. That is, when the phosphatability is not good, sufficient baking property can not be ensured.
In the steel sheet for automobiles, it is also required to subject the present part to a spot welding, so that the spot weldability of steel sheet becomes important.
In the press forming, there may be caused the seizing between the steel sheet and the press mold, or a so-called galling phenomenon. Such a galling unfavorably causes the damage of the mold, considerable degradation of commercial value of the pressed parts and the like.
In the automobile industry, one-side surface treated steel sheets have hitherto been used as a body plate of an automobile exposed to severer corrosion environment, wherein the inner surface of the steel sheet is a plated or organic coating surface and the outer surface thereof is a cold rolled surface. Even in the outer surface of the body plate, however, rusting or blistering is caused due to the collision with gravels, pebbles and so on. Therefore, both-side surface plated steel sheets have lately been used as the body plate.
Since the steel sheet for automobile is subjected to various press forming prior to the assembling into the automobile body, it is required to have an excellent deep drawability. However, the galvanized steel sheets usually used for the automobile are apt to be seized to the press mold in the press forming due to the presence of galvanized coating as compared with the usual cold rolled steel sheet, and are poor in the deep drawability.
The investigations on deep drawing are made from two viewpoints of steel sheet and drawing technique. However, requirements for the steel sheet tend to become high-grade and deversified together with the accurate increase and complication of the product. Particularly, this tendency is strong in the cold rolled steel sheet for automobile.
For instance, plural pressed parts are spot-welded in the assembling of the vehicle body at the present. Therefore, it is strongly demanded to reduce the number of spot-welded points by oversizing the pressed part or making these pressed parts into one body. On the other hand, the car design becomes more complicated in compliance with the various needs, and consequently difficult molding parts increase in the usual cold rolled steel sheet. In order to satisfy these needs, it is necessary to use cold rolled steel sheets having an improved press formability as compared with the usual cold rolled steel sheet.
In the actual press forming, the mechanical properties (r-value, E1, n-value) of steel sheet have hitherto been used as an evaluation standard of press formability, but they are not still insufficient. For instance, the press formability is also largely influenced by the surface roughness of steel sheet, the lubricating oil and the like.
There are some well-known techniques showing an influence of surface roughness of steel sheet upon press formability and so on. For example, Plasticity and Work, Vol. 3, No. 14 (1962-3) discloses that when using a high viscosity lubricating oil, the drawability is most improved at the steel sheet surface roughness of about several .mu.m. On the other hand, Japanese Patent Application Publication No. 59-34,441 discloses that the appearance after painting and press formability are more improved by subjecting a cold rolled steel sheet to a skin pass rolling through a dull roll having a center-line average surface roughness of Ra=2.8 (.mu.m) and peak number of PPI=226 as a roll surface roughness.
These well-known techniques are excellent in view of the improvement of press formability, but have a drawback that the surface roughness of steel sheet should be controlled to a certain level.
In Japanese Patent laid open No. 54-97,527 is disclosed a method wherein a cold rolled steel sheet having an improved phosphatability can be produced by subjecting to a skin pass rolling through a roll having PPI=150 as a roll surface roughness. This method provides an excellent phosphatability but does not develop an effect on press formability. In general, the phosphatability is required for the steel sheet used in automobiles, and also the press formability and distinctness of image after painting (DOI) become necessarily important.
The aforementioned conventional techniques do not teach nor suggest a method of producing cold rolled steel sheets and plated steel sheets having an optional surface roughness (Ra, PPI) and excellent press formability, phosphatability, weldability and resistance to galling.